Method for judging quality of lithium nickel composite oxide and positive electrode using lithium nickel composite oxide

ABSTRACT

A positive electrode active material quality judgment method that can easily and accurately judge the quality of a positive electrode active material used in a non-aqueous electrolyte secondary cell without having to complete the positive electrode. The positive electrode active material quality judgment method includes: heating a positive electrode active material mainly made of a lithium nickel composite oxide to a temperature x (° C.) of 200° C. or higher and 400° C. or lower; measuring the amount of carbon dioxide gas generated from the heating; and the positive electrode active material as a suitable positive electrode active material when the positive electrode active material satisfies formulas 3 and 4:
 
 y &lt;(1.31 x −258)/1000000(200≦ x &lt;300)  formula 3
 
 y &lt;1.20 x −225/1000000(300≦ x ≦400)  formula 4
 
where x is the heating temperature x (° C.) and y is the amount of carbon dioxide gas (mole/g) generated per 1 g of the positive electrode active material in the heating to the heating temperature x (° C.).

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a lithium nickel composite oxideserving as a positive electrode active material and a method for judgingthe quality of a positive electrode using this substance.

2) Description of the Related Art

Non-aqueous electrolyte secondary cells, for their high energy densityand high capacity, are widely used as power sources for mobileappliances. Conventionally, as the positive electrode active materialused for the non-aqueous electrolyte secondary cells, lithium cobaltcomposite oxide (LiCoO₂), which excels in discharge property, has beenused.

However, an increasing need for further enhancement of cell capacity andan increase in cell cost due to rising prices of cobalt have drawnattention to lithium nickel composite oxide Li_(a)Ni_(x)M_(1-x)O₂ (whereM is at least one selected from Co, Al, Zr, Ti, Mg, and Mn, 0.9≦a≦1.1,and 0.5≦x≦1) as the positive electrode active material of non-aqueouselectrolyte secondary cells.

However, the lithium nickel composite oxide still possess problems to besolved, and cells using the lithium nickel composite oxide have a higheroccurrence rate of defectiveness than cells using lithium cobaltcomposite oxide.

As an example of the problems to be solved, non-patent document 1(“Abstracts of Speeches at the 47th Battery Symposium,” pp. 326-327)reports that if a lithium nickel composite oxide exposed to atmosphereis used to constitute a non-aqueous electrolyte secondary cell, cellswelling occurs from high-temperature preservation.

Non-patent document 1 gives a possible cause for such cell swelling asfollows. If the lithium nickel composite oxide is exposed to atmosphere,the lithium ions in the lithium nickel composite oxide react withmoisture in atmosphere whereby a highly reactive lithium hydroxideoccurs, which in turn reacts with carbon dioxide in atmosphere to resultin lithium carbonate (Li₂CO₃). Also, the moisture contained in theatmosphere-exposed lithium nickel composite oxide decomposes LiPF₆serving as electrolytic salt inside the cell to generate hydrofluoricacid (HF). This in turn decomposes the lithium carbonate (Li₂CO₃),thereby generating carbon dioxide gas inside the cell. The gas generatedinside the cell possibly remains between the positive and the negativeelectrodes to the detriment of the opposing condition thereof, resultingin a decrease in cell capacity.

The generation of lithium hydroxide possibly causes a decrease in cellcapacity also in such a respect that the amount of lithium nickelcomposite oxide that contributes to charge/discharge decreases.

Incidentally, as a method for solving these problems associated with thelithium nickel composite oxide, carrying out the whole process of cellproduction under conditions without exposure to atmosphere, such as in adry air atmosphere and an inactive gas atmosphere, are contemplated.However, this method causes a significant increase in production cost.Therefore, this method is not practical.

In view of these circumstances, such a method is conventionally employedthat tests for cell swelling, a decrease in cell capacity, and the likeare carried out after a cell is complete, and when the cell is judged tobe unsuitable, all the cells of the same production lot are discarded.However, this method may significantly degrade the production yield, andthe decreased production yield causes the problem of raising the priceof the complete cell.

SUMMARY OF THE INVENTION

Thus, cell swelling, a decrease in cell capacity, and the like occur tothe non-aqueous electrolyte secondary cell using lithium nickelcomposite oxide as the positive electrode active material. Eliminatingin advance the causes for these problems significantly enhances theproduction yield. This necessitates a judgment method that easilypredicts occurrence of the problems before the cell is complete, butsuch method is still not established.

The present invention has been accomplished in order to solve the aboveproblems encountered when using lithium nickel composite oxide as thepositive electrode active material.

It is a first object of the present invention to provide a method(positive electrode active material quality judgment method) that caneasily judge the quality of a positive electrode active material mainlymade of lithium nickel composite oxide without having to complete thepositive electrode.

It is a second object of the present invention to provide a method(positive electrode quality judgment method) that can easily judge thequality of a positive electrode using lithium nickel composite oxide asthe positive electrode active material without having to complete thecell.

(1) In order to accomplish the above-mentioned objects, a qualityjudgment method for a positive electrode active material having alithium nickel composite oxide according to a first aspect of thepresent invention is configured as follows.

The quality judgment method for a positive electrode active materialhaving a lithium nickel composite oxide includes the steps of: a heatingstep of heating the positive electrode active material to a temperaturex (° C.) of 200° C. or higher and 1500° C. or lower in an inactive gasatmosphere; a measuring step of measuring the amount of carbon dioxidegas generated from the heating; and a judging step of judging thepositive electrode active material as a suitable positive electrodeactive material when the positive electrode active material satisfiesformulas 1 and 2:y<(0.27x−51)/1000000(200≦x<400)  formula 1y<57/1000000(400≦x≦1500)  formula 2

where x is the heating temperature x (° C.) and y is the amount ofcarbon dioxide gas (mole/g) generated per 1 g of the positive electrodeactive material heated to the heating temperature x (° C.).

When a lithium nickel composite oxide exposed to atmosphere is heated to200° C. or higher and 1500° C. or lower in an inactive gas atmosphere,carbon dioxide gas is generated. The present inventors have discovered acause for this: The lithium ions in the lithium nickel composite oxidereact with moisture in atmosphere whereby lithium hydroxide isgenerated, which in turn reacts with carbon dioxide gas in atmosphere,generating a thermally decomposing carbonate compound, which isdescribed later.

The series of reactions to generate the thermally decomposing carbonatecompound are reactions to decrease the amount of the lithium nickelcomposite oxide contributive to charge/discharge, and therefore,occurrence of these reactions decreases the discharge capacity. Also,when a cell with a lithium nickel composite oxide that has experiencedthese reactions is preserved at a high temperature, particularly at 60°C. or higher, then the thermally decomposing carbonate compound isdecomposed to generate carbon dioxide gas, thereby causing the cell toswell in a large degree. Hence, making the content of the thermallydecomposing carbonate compound small is essential in obtaining a cell ofgood quality, and whether the content of the decomposing carbonatecompound is large or small can be an indicator by which to judge thequality of the positive electrode active material.

That is, there is a correlation between cell performance and the amountof carbon dioxide gas (hereinafter referred to as carbon dioxide gasamount) that occurs when the lithium nickel composite oxide is heated to200° C. or higher and 1500° C. or lower in an inactive gas atmosphere.Therefore, by sampling a part of a lithium nickel composite oxide of thesame production lot (i.e., production under the same conditions),heating the sample at the above-mentioned temperature range, measuringthe resulting carbon dioxide gas amount, and introducing the carbondioxide gas amount in formulas 1 and 2, whether the lithium nickelcomposite oxide of this lot is a suitable product can be judged.

The results of this judgment can be utilized in the following manner.

(1) By selecting and using a lithium nickel composite oxide judged to besuitable for the positive electrode active material by the above qualityjudgment, a high quality positive electrode can be produced.

(2) When a lithium nickel composite oxide is judged to be unsuitable bythe above quality judgment method, this indicates that there areproblems with the synthesis conditions, selection of raw material, andpreservation of the lithium nickel composite oxide, and therefore, someexamination for these problems may be carried out to eliminate them.

This configuration will be further described. When the lithium nickelcomposite oxide is heated in an active gas atmosphere containing oxygengas in the heating in the heating step, a minute amount of organicsubstance that remains on the lithium nickel composite oxide and a testcontainer combusts (reacts with oxygen) to generate carbon dioxide gas.This disables the amount of the carbon dioxide gas caused by thethermally decomposing carbonate compound to be measured accurately,thereby necessitating the lithium nickel composite oxide to be heated inan inactive gas atmosphere.

As the inactive gas, argon gas or nitrogen gas is preferably used, andmore preferably, argon gas is used.

If the heating temperature x (° C.) is lower than 200° C., the amount ofthe carbon dioxide gas generated is too small, which makes it difficultto judge the quality of the lithium nickel composite oxide. In view ofthis, the heating temperature x (° C.) is 200° C. or higher. If theheating temperature x (° C.) exceeds 1500° C., lithium carbonate that iscaused by the lithium source and that is not reactive in the baking stepis thermally decomposed to generate carbon dioxide gas, which also makesit difficult to judge the quality of the lithium nickel composite oxide.In view of this, the heating temperature x (° C.) is 1500° C. or lower.

In order to further enhance the reliability of the judgment, the heatingtemperature x (° C.) is preferably 400° C. or higher, at which thegeneration of carbon dioxide gas is almost complete. In view of heatingcosts, the upper limit of the heating temperature x (° C.) is preferably800° C. More preferably, the heating temperature is 450±50° C.

As the method for measuring the carbon dioxide gas amount, gaschromatography is preferably used in that the gas generation amount canbe analyzed easily, in a short time, and accurately. The carbon dioxidegas amount may be measured by thermal gravimetry analysis (TGA), whichmeasures a change in mass before and after heating.

Also in the first aspect of the present invention, the heating step mayinclude: putting the positive electrode active material in a stainlesssteel reaction tube filled with the inactive gas: heating the reactiontube to 200° C. or higher and 1500° C. or lower in order to generatecarbon dioxide gas; and measuring the amount of the carbon dioxide gasby gas chromatography. This configuration is preferable in thatsubstitution to the inactive atmosphere is facilitated, and the carbondioxide gas that is generated from heating the lithium nickel compositeoxide can be captured accurately.

While the positive electrode active material is mainly made of lithiumnickel composite oxide, it is also possible to contain, in addition tothe lithium nickel composite oxide, a known positive electrode activematerial other than the lithium nickel composite oxide, examplesincluding lithium cobalt composite oxide and spinel lithium manganesecomposite oxide.

(2) In order to accomplish the above-mentioned objects, a qualityjudgment method for a positive electrode for a non-aqueous electrolytesecondary cell having a positive electrode mixture layer containing alithium nickel composite oxide and polyvinylidene fluoride according toa second aspect of the present invention is configured as follows.

The quality judgment method for a positive electrode for a non-aqueouselectrolyte secondary cell having a positive electrode mixture layercontaining a lithium nickel composite oxide and polyvinylidene fluorideincludes the steps of: a sampling step of sampling the positiveelectrode mixture layer from the positive electrode; a heating step ofheating the sampled positive electrode mixture layer to a temperature x(° C.) of 200° C. or higher and 400° C. or lower; a measuring step ofmeasuring the amount of carbon dioxide gas generated from the heating;and a judging step of judging the positive electrode as a suitablepositive electrode for the non-aqueous electrolyte secondary cell whenthe positive electrode active material satisfies formulas 3 and 4:y<(1.31x−258)/1000000(200≦x<300)  formula 3y<1.20x−225/1000000(300≦x≦400)  formula 4

where x is the heating temperature x (° C.) and y is the amount ofcarbon dioxide gas (mole/g) generated per 1 g of the positive electrodeactive material heated to the heating temperature x (° C.).

Even in the case of using the lithium nickel composite oxide selectedaccording to the first aspect of the present invention, the lithiumnickel composite oxide and impurities contained therein may react withmoisture and carbon dioxide by, for example, contact with atmosphere inthe series of steps for completing the positive electrode including:preparing a positive electrode mixture by mixing a binding agent andother additives in the lithium nickel composite oxide; and applying themixture onto a positive electrode plate and rolling the plate. Thisnecessitates a judgment method for evaluating the suitability of thecomplete positive electrode in addition to judging the quality of thelithium nickel composite oxide as the positive electrode active materialbefore the positive electrode is complete. The second aspect of thepresent invention relates to a positive electrode quality judgmentmethod that enables judgment as to whether the complete positiveelectrode is a suitable product.

A binding agent is necessary in applying the positive electrode activematerial onto the positive electrode core material, and as this bindingagent, polyvinylidene fluoride is usually used. The present inventorsremoved a positive electrode mixture layer (active material layer) outof a lithium nickel composite oxide positive electrode usingpolyvinylidene fluoride as the binding agent, and heated the positiveelectrode mixture layer. It has been found that a larger amount ofcarbon dioxide gas is generated than when heating the lithium nickelcomposite oxide alone. The present inventors have further found that thelarge amount of carbon dioxide gas is not caused by moisture containedin the binding agent and solvent or by moisture contained in atmosphere,but by the existence of the polyvinylidene fluoride itself.

Specifically, polyvinylidene fluoride is decomposed by heating togenerate hydrogen fluoride (HF). This hydrogen fluoride is generated asa side reaction product of synthesis of the thermally decomposingcarbonate compound and the lithium nickel composite oxide, and reactswith lithium carbonate that remains in the lithium nickel compositeoxide to generate carbon dioxide gas. It has been found that the carbondioxide gas amount increases in this manner.

The present inventors have also found that when the positive electrodemixture layer containing polyvinylidene fluoride is heated at hightemperature, the hydrogen fluoride derived from the polyvinylidenefluoride becomes more influential, which makes the judgment of thequality of the positive electrode difficult, whether the carbon dioxidegas amount is large or small.

On the basis of the above findings, the thermal decompositiontemperature x (° C.) of the positive electrode mixture layer accordingto the second aspect of the present invention is specified as 200° C. orhigher and 400° C. or lower. This is because when heating within therange of 200° C. or higher and 400° C. or lower, the generated amount ofthe carbon dioxide gas caused by hydrogen fluoride is small and theamount of the carbon dioxide gas derived from the thermally decomposingcarbonate compound is sufficiently large. Also within this temperaturerange, there is a sufficient correlation observed between the amount ofthe carbon dioxide gas that is generated from heating and cellperformance.

Therefore, by removing the positive electrode mixture layer (layer madeof the positive electrode active material, binding agent, conductiveagent added as necessary, and the like) out of a part of the positiveelectrode prepared under the same conditions or out of an extra portionthat occurs during cutting for a necessary size (sampling step), heatingthe positive electrode mixture layer at a temperature within the abovetemperature range, measuring the amount of the resulting carbon dioxidegas, and introducing the heating temperature and the carbon dioxide gasgeneration amount in formulas 3 and 4, whether the prepared positiveelectrode is a suitable product can be judged. By selecting and using apositive electrode judged as a good product by this judgment, a highquality non-aqueous electrolyte secondary cell is produced with a goodproduction yield.

The measuring step is preferably carried out by gas chromatography,similarly to the first aspect of the present invention.

The heating in the heating step is carried out in an inactive gasatmosphere, and as the inactive gas, argon gas or nitrogen gas ispreferably used, and more preferably, argon gas is used.

The heating temperature x (° C.) is preferably 300±50° C., morepreferably 300±10° C. At a heating temperature of around 300° C., thegeneration amount of the carbon dioxide gas caused by hydrogen fluoridebecomes relatively small to a negligible degree, thereby enhancing thereliability of the judgment.

Also in the second aspect of the present invention, the heating step mayinclude: putting the positive electrode mixture layer in a stainlesssteel reaction tube filled with the inactive gas; and heating thereaction tube to the heating temperature x (° C.) in order to generatecarbon dioxide gas, and the measuring step may be a step of measuringthe amount of the carbon dioxide gas generated in the reaction tube bygas chromatography. In this case, the heating temperature x (° C.) is200° C. or higher and 400° C. or lower, preferably 300±50° C.

This configuration is preferable in that substitution to the inactiveatmosphere is facilitated, and the carbon dioxide gas that is generatedfrom heating the positive electrode mixture layer can be capturedaccurately.

Also in the second aspect of the present invention, similarly to thefirst aspect, while the positive electrode active material is mainlymade of lithium nickel composite oxide, it is also possible to contain,in addition to the lithium nickel composite oxide, a known positiveelectrode active material other than the lithium nickel composite oxide,examples including lithium cobalt composite oxide and spinel lithiummanganese composite oxide. It should be noted, however, that if thecontent of the lithium nickel composite oxide is too small, theadvantageous effects (reduction in cost and increase in capacity) ofusing the lithium nickel composite oxide as the positive electrodeactive material cannot be sufficiently exhibited. In view of this, themass percentage of the good quality lithium nickel composite oxide inthe total mass of the positive electrode active material is 50 mass % ormore, more preferably 75 mass % or more.

As has been described hereinbefore, with the positive electrode activematerial quality judgment method according to the present invention,whether a lithium nickel composite oxide is a suitable lithium nickelcomposite oxide usable as the positive electrode active material can bejudged easily and accurately on the level where the lithium nickelcomposite oxide is a raw material. Also with the positive electrodequality judgment method according to the present invention, the qualityof a positive electrode containing lithium nickel composite oxide can bejudged easily and accurately before assembly of the cell. By using thequality judgment methods according to the present invention, a highquality positive electrode and a non-aqueous electrolyte secondary cellhaving desired performance can be produced with a good yield.

BRIEF DESCRIPTION OF THE DRAWING

The present invention as defined in the claims can be better understoodwith reference to the text and to the following drawings, as follows:

FIG. 1 is a graph showing the results of the thermal decomposition bygas chromatography of sodium hydrogen carbonate and lithium carbonate;

FIG. 2 is a graph showing the results of the thermal decomposition bygas chromatography of a positive electrode active material (lithiumnickel composite oxide);

FIG. 3 is a graph showing the result of the thermal decomposition by gaschromatography of a positive electrode active material (lithium nickelcomposite oxide) exposed to atmosphere for 10 days;

FIG. 4 is a graph showing the theoretical calculation value of theamount of a carbonate compound contained in the positive electrodeactive material;

FIG. 5 is a graph showing a relation between the amount of cell swellingand the amount of carbon dioxide gas occurring at 500° C.;

FIG. 6 is a graph showing the results of the thermal decomposition bygas chromatography of a positive electrode mixture layer contained in acomplete positive electrode (lithium nickel composite oxide positiveelectrode);

FIG. 7 is a graph showing the results of the thermal decomposition bygas chromatography of a positive electrode mixture layer contained in apositive electrode that uses lithium cobalt composite oxide as thepositive electrode active material;

FIG. 8 is a graph showing the results of the thermal decomposition bygas chromatography of a positive electrode mixture layer contained in apositive electrode of different exposure conditions;

FIG. 9 is a graph that compares a carbon dioxide gas generation amountin the case of heating the positive electrode active material with acarbon dioxide gas generation amount in the case of heating the positiveelectrode mixture layer contained in the complete cell;

FIG. 10 is a graph showing a relation between the amount of cellswelling and the amount of the carbon dioxide gas that occurs fromheating the positive electrode mixture layer contained in the completecell to 300° C.;

FIG. 11 is a graph showing a relation between the kind of the atmosphereduring heating and the carbon dioxide gas generation amount in thelithium nickel composite oxide; and

FIG. 12 is a graph showing a relation between the kind of the atmosphereduring heating and the carbon dioxide gas generation amount in thepositive electrode mixture layer.

DETAILED DESCRIPTION OF THE INVENTION

The quality judgment method of the present invention will be proved byvarious experiments.

EXPERIMENT 1

Various experimental example cells were prepared and analyzed for theirproperties in order to reveal a relation between cell preparationconditions and cell performance.

EXPERIMENTAL EXAMPLE 1

<Preparation of the Positive Electrode>

Nickel, cobalt, and aluminum were co-precipitated with sulfate to havenickel-cobalt-aluminum hydroxide. Lithium hydroxide was added to thenickel-cobalt-aluminum hydroxide, followed by baking at 700° C., thusobtaining lithium nickel composite oxide containing cobalt and aluminum(LiNi_(0.8)Co_(0.15)Al_(0.05)O₂).

The element content of the lithium nickel composite oxide was analyzedby ICP-AES (Inductively Coupled Plasma—Atomic Emission Spectrometry),and LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ was confirmed.

Next, using a dry air atmosphere having a dew point of −40° C. or lower,95 mass parts of the lithium nickel composite oxide, 2.5 mass parts ofcarbon powder as a conducting agent, 2.5 mass parts of polyvinylidenefluoride (PVdF) as a binding agent, and N-methyl-2-pyrrolidone (NMP)were mixed together, thus preparing a positive electrode active materialslurry. This positive electrode active material slurry was applied toboth surfaces of a positive electrode current collector made of aluminumin an atmosphere of 43% relative humidity and 25° C. temperature, andthen dried. Then, the resulting product was rolled with a compressiveroller in a dry air atmosphere having a dew point of −40° C. or lower,thus completing a positive electrode.

<Preparation of the Negative Electrode>

97.5 mass parts of natural graphite as the negative electrode activematerial, 1.5 mass parts of styrene-butadiene rubber (SBR), 1 mass partof carboxy methylcellulose (CMC) as a thickening agent, and pure waterwere mixed together, thus preparing a negative electrode active materialslurry. This negative electrode active material slurry was applied toboth surfaces of a negative electrode current collector made of copper,followed by drying. Then, the dried electrode plate was rolled with acompressive roller, thus completing a negative electrode.

<Preparation of the Electrode Assembly>

In the above atmosphere, the positive electrode and the negativeelectrode were wound with a separator made of a polypropylene porousfilm therebetween and then pressed, thus preparing a flat electrodeassembly.

<Preparation of the Non-aqueous Electrolyte>

Ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate weremixed together at a volume ratio of 2:5:3 (25° C.), and then LiPF₆ aselectrolytic salt was dissolved therein at a rate of 1.2M (mol/liter),thus obtaining a non-aqueous electrolyte.

<Assembly of the Cell>

A commercial aluminum laminate material was prepared. This aluminumlaminate material was folded, and the side edges thereof were thermallyfused, thus forming a bag-like outer casing having an electrode assemblystorage space. Then, the above flat electrode assembly was inserted intothe storage space, followed by 2.5 hours of vacuum drying at 105° C.

Next, in a dry box of argon atmosphere, the flat electrode assembly andthe non-aqueous electrolyte were housed in the storage space. Then, theinterior of the outer casing was depressurized to immerse thenon-aqueous electrolyte inside the separator, and the opening portion ofthe outer casing was sealed, thus preparing a 62 mm high, 35 mm wide,3.6 mm thick non-aqueous electrolyte secondary cell accordingexperimental example 1 with a theoretical capacity of 800 mAh.

EXPERIMENTAL EXAMPLE 2

A non-aqueous electrolyte secondary cell according to experimentalexample 2 was prepared in the same manner as in experimental example 1except that after the positive electrode was complete, the positiveelectrode was left in a dry box having a dew point of −40° C. or lowerunder a 25° C. condition for 10 days (dry air exposure), and this wasused as the positive electrode. In experimental example 1, this productwas used to complete the positive electrode immediately after thelithium nickel composite oxide was prepared, and immediately thereafter,this complete positive electrode was used to complete the cell.

EXPERIMENTAL EXAMPLE 3

A non-aqueous electrolyte secondary cell according to experimentalexample 3 was prepared in the same manner as in comparative example 1except that after the positive electrode was complete, the positiveelectrode was left in a dry box having a dew point of −40° C. or lowerunder a 25° C. condition for 30 days, and this was used as the positiveelectrode.

EXPERIMENTAL EXAMPLE 4

A non-aqueous electrolyte secondary cell according to experimentalexample 4 was prepared in the same manner as in experimental example 1except that after the positive electrode was complete, the positiveelectrode was left in an atmosphere of 25° C. temperature and 43%relative humidity for 3 hours (atmosphere exposure), and this was usedas the positive electrode.

EXPERIMENTAL EXAMPLE 5

A non-aqueous electrolyte secondary cell according to experimentalexample 5 was prepared in the same manner as in experimental example 1except that after the positive electrode was complete, the positiveelectrode was left in an atmosphere of the above conditions for a day(atmosphere exposure), and this was used as the positive electrode.

EXPERIMENTAL EXAMPLE 6

A non-aqueous electrolyte secondary cell according to experimentalexample 6 was prepared in the same manner as in experimental example 1except that after the positive electrode was complete, the positiveelectrode was left in an atmosphere of the above conditions for 3 days(atmosphere exposure), and this was used as the positive electrode.

EXPERIMENTAL EXAMPLE 7

A non-aqueous electrolyte secondary cell according to experimentalexample 7 was prepared in the same manner as in experimental example 1except that after the positive electrode was complete, the positiveelectrode was left in an atmosphere of the above conditions for 10 days(atmosphere exposure), and this was used as the positive electrode.

EXPERIMENTAL EXAMPLE 8

A non-aqueous electrolyte secondary cell according to experimentalexample 8 was prepared in the same manner as in experimental example 1except that after the positive electrode was complete, the positiveelectrode was left in an atmosphere of the above conditions for 10 days(atmosphere exposure), and then left in a dry box having a dew point of−40° C. or lower under a 25° C. condition for 10 days (dry airexposure), and this was used as the positive electrode.

<Cell Swelling Amount Measurement Test>

Two cells prepared under the conditions of each of the experimentalexamples were charged at a constant current of 1.0 It (800 mA) to avoltage of 4.2 V, then at a constant voltage of 4.2 V to a current of0.05 It (40 mA), followed by measurement of cell thickness. Then, thecharged cells were left in a thermostatic chamber of 85° C. for 3 hours,and cell thickness was measured immediately after the cells were pickedout (thickness immediately after picking out). Then, the cells were leftat 25° C. for 1 hour, and cell thickness after cooling was measured(thickness after cooling). The amount of cell swelling immediately afterpreservation and the amount of cell swelling after cooling werecalculated. The results (average values) are shown in Table 1.

<Charge/Discharge Property Test>

Two cells prepared under the conditions of each of the experimentalexamples were charged at a constant current of 1.0 It (800 mA) to avoltage of 4.2 V, then at a constant voltage of 4.2 V to a current of0.05 It (40 mA), followed by measurement of cell thickness. Then, thecells were discharged at a constant current of 1.0 It (800 mA) to avoltage of 2.5 V, followed by measurement of discharge capacity. Also aninitial efficiency was calculated from the following formula. Theresults (average values) are shown in Table 1.Initial efficiency (%)=discharge capacity/charge capacity×100

The charge capacity and the discharge capacity are represented byrelative values with the results in experimental example 1 assumed to be100.

<Discharge Load Property Test>

Two cells prepared under the conditions of each of the experimentalexamples were charged at a constant current of 1.0 It (800 mA) to avoltage, of 4.2 V, then at a constant voltage of 4.2 V to a current of0.05 It (40 mA), followed by measurement of cell thickness. Then, thecells were discharged at a constant current of 1.0 It (800 mA) to avoltage of 2.5 V, followed by measurement of discharge capacity (1.0 Itdischarge capacity). Then, the cells were charged again under the aboveconditions and discharged at a constant current of 0.2 It (160 mA) to avoltage of 2.5 V, followed by measurement of discharge capacity (0.2 Itdischarge capacity). Also a discharge load property was calculated fromthe following formula. The results (average values) are shown in Table1.Discharge load (%)=0.2 It discharge capacity/1.0 It dischargecapacity×100

TABLE 1 Amount of cell swelling (mm) Immediately Exposure after pickingCharge Discharge Initial Discharge load conditions out After coolingcapacity (%) capacity (%) efficiency (%) property (%) ExperimentalExample 1 None 0.73 0.26 100.0 100.0 88.8 106.7 Experimental Example 2Dry air: 10 0.92 0.36 99.6 100.1 89.1 106.6 days Experimental Example 3Dry air: 30 1.17 0.43 100.2 99.7 88.5 107.0 days Experimental Example 4Atmosphere: 3 Hrs 1.12 0.46 100.5 100.4 88.9 106.9 Experimental Example5 Atmosphere: 1 1.89 0.69 100.2 98.8 88.4 107.8 day Experimental Example6 Atmosphere: 3 3.97 1.25 99.0 96.9 88.1 108.2 days Experimental Example7 Atmosphere: 10 4.43 1.61 96.0 90.0 85.9 110.2 days ExperimentalExample 8 Atmosphere: 10 4.89 1.67 95.9 89.2 85.5 110.5 days Dry air: 10days

Table 1 shows that experimental example 1, which experienced noexposure, was small in cell swelling amount compared with experimentalexamples 2 and 3, which experienced dry air exposure, experimentalexamples 4 to 7, which experienced atmosphere exposure, and experimentalexample 8, which experienced dry air exposure after atmosphere exposure.Table 1 also shows that the longer the dry air exposure time is, thelarger the cell swelling amount tends to be (see experimental examples 2and 3), and that the longer the atmosphere exposure time is, the largerthe cell swelling amount tends to be (see experimental examples 4 to 7).Table 1 also shows that in the cases of the same exposure time, the cellswelling amount is larger in the cases of atmosphere exposure than inthe cases of dry air exposure (see experimental examples 2, 3, 6, and7).

Table 1 also shows that experimental examples 7 and 8 are significantlyinferior to experimental example 1 in that the charge capacity isrespectively 96.0% and 95.9% of that of experimental example 1, thedischarge capacity is respectively 90.0% and 89.2% of that ofexperimental example 1, the initial efficiency is respectively 85.9% and85.5% when that of experimental example 1 is 88.8%, and the dischargeload property is respectively 110.2% and 110.5% when that ofexperimental example 1 is 106.7%.

These results are possibly due to adverse affects of moisture and carbondioxide in atmosphere from exposure. When the complete positiveelectrode is exposed to atmosphere, moisture and carbon dioxide inatmosphere react with the lithium ions in the lithium nickel compositeoxide, which is the positive electrode active material, resulting inoccurrence of a plurality of reaction products that cause cell swelling.From the result that swelling occurs also due to dry air exposure, thisreaction possibly occurs, though slightly, in a dry air atmospherehaving a dew point of −40° C. or lower.

Since the reaction products resulting from atmosphere exposure are notsubstances contributive to charge/discharge, the occurrence of thereaction products possibly reduces the amount of the active material,resulting in degradation of the charge capacity and the dischargecapacity. Further, the occurrence of the reaction products possiblydegrades the conductivity of the positive electrode, resulting indegradation of the discharge load property.

EXPERIMENT 2

The results shown in Table 1 reveal that the moisture incorporated inthe positive electrode active material and the positive electrode is acause of the degradation of cell performance. It is therefore possibleto judge whether the quality of the positive electrode active materialand the positive electrode is good or bad by measuring the amount ofmoisture contained in the positive electrode active material and thepositive electrode. This was studied in experiment 2.

<Measurement of the Amount of Moisture>

A lithium nickel composite oxide (positive electrode active material)was prepared in the same manner as in experimental example 1. Thispositive electrode active material was exposed under conditions shown inTable 2, and the amount of moisture contained in the exposed positiveelectrode active material was measured by the Karl Fischer method. Alsoa positive electrode was prepared in the same manner as in experimentalexample 1 and exposed under conditions shown in Table 2. Then, apositive electrode mixture layer (layer made of the positive electrodeactive material, binding agent, and conductive agent) was stripped offthe positive electrode, and the amount of moisture contained in thepositive electrode mixture layer was measured by the Karl Fischermethod. The results are shown in Table 2.

TABLE 2 Amount of moisture (ppm) Positive electrode Positive electrodeNo. active material a mixture layer b 1 No exposure 290 597 2 Dry air:10 days 292 533 3 Dry air: 30 days 325 874 4 Atmosphere: 3 hrs 612 — 5Atmosphere: 1 day 1329 — 6 Atmosphere: 3 days 2256 3552  7 Atmosphere: 5days 2528 — 8 Atmosphere: 10 days 3708 4299  9 Atmosphere: 10 days 3244996 Dry air: 10 days —: Not measured.

From Table 2, it has been found that for both the positive electrodeactive material and the positive electrode mixture layer, the longer thetime for atmosphere exposure is, the larger the amount of moisture tendsto be. Also it has been found that in the case of 10 days of dry airexposure after 10 days of atmosphere exposure for both the positiveelectrode active material and the positive electrode mixture layer, theamount of moisture was smaller than in the case of 10 days of atmosphereexposure alone. However, in Table 1, experimental example 8, whichexperienced 10 days of dry air exposure after 10 days of atmosphereexposure, is inferior in various cell properties to experimental example7, which experienced 10 days of atmosphere exposure alone. Also it hasbeen found that compared with experimental example 1, the cell swellingamount is large in experimental example 2, which experienced 10 days ofdry air exposure, and experimental example 3, which experienced 30 daysof dry air exposure.

These results show that it is impossible to judge the quality of thepositive electrode active material and the positive electrode by themoisture content. The result for No. 9 shown in Table 2 is possiblybecause the dry air exposure removed part of the moisture adsorbed ontothe positive electrode active material and the positive electrode duringthe atmosphere exposure.

EXPERIMENT 3

In experiment 3, a study was conducted on a method for judging thequality of the positive electrode active material and the positiveelectrode by using an indicator other than the moisture content.

<Concept>

If lithium hydroxide reacts with carbon dioxide gas in the atmospheregas, a lithium carbonate compound is generated, and it is possible thatthis lithium carbonate compound is decomposed at high temperaturepreservation to generate carbon dioxide gas, thereby causing the amountof cell swelling to increase and the conductivity of the positiveelectrode to degrade. The present inventors focused on the lithiumcarbonate compound contained in the lithium nickel composite oxide andthe lithium nickel composite oxide positive electrode, and speculatedthat the quality of the positive electrode active material and thepositive electrode may be judged by measuring the amount of the carbondioxide gas that is derived from the lithium carbonate compoundgenerated from heating the positive electrode active material.

Possible reaction products (lithium carbonate compounds) of the lithiumhydroxide carbon dioxide gas include lithium carbonate (Li₂CO₃) andlithium hydrogen carbonate (LiHCO₃). In experiment 3, patterns ofthermal decomposition of these compounds were analyzed. It should benoted that while the lithium carbonate was commercially available, thelithium hydrogen carbonate was not currently commercially available anddifficult to obtain, and so instead of the lithium hydrogen carbonate,sodium hydrogen carbonate (NaHCO₃), which has a similar structure tothat of the lithium hydrogen carbonate (LiHCO₃), was used. Thecommercial unavailability of the lithium hydrogen carbonate ispresumably because the lithium hydrogen carbonate is an unstablesubstance.

The method for this experiment is as follows. Two argon gas-filledreaction tubes made of SUS (Stainless Used Steel) were prepared, and thelithium carbonate and the sodium hydrogen carbonate each were put into adifferent one of the tubes. Then the tubes were thermally treated in anelectric furnace, followed by measurement of the amount of the resultingcarbon dioxide gas by gas chromatography. The gas chromatographyapparatus used was GC-14B, available from Shimadzu Corporation. Thismethod will be hereinafter referred to as thermal decomposition-gaschromatography method.

The results of the thermal decomposition-gas chromatography method areshown in FIG. 1. FIG. 1 shows that by heating at 200° C. or higher, 1mole of the sodium hydrogen carbonate generates approximately 0.45 moleof carbon dioxide gas.

The sodium hydrogen carbonate is possibly decomposed in the mannerrepresented by the following reaction formula and generates carbondioxide gas, and therefore, 0.5 mole of carbon dioxide gas is generatedper 1 mole of the sodium hydrogen carbonate.2NaHCO₃→Na₂CO₃+H₂O+CO₂↑

Thus, by heating at 200° C. or higher, 90% of the sodium hydrogencarbonate is decomposed.

FIG. 1 also shows that the lithium carbonate is hardly decomposed byheating at 100 to 500° C. These experimental results definitely showthat the carbon dioxide gas that is generated from heating theatmosphere-exposed lithium nickel composite oxide at the condition of500° C. or lower is derived from a substance other than the lithiumcarbonate. When almost no carbon dioxide gas is generated during heatingunder this temperature condition, the lithium carbonate compoundcontained in the lithium nickel composite oxide is assumed to be lithiumcarbonate. Validity of this assumption was further studied in experiment4.

EXPERIMENT 4

A lithium nickel composite oxide was prepared in the same manner as inexperimental example 1. This lithium nickel composite oxide was exposedfor a predetermined period of time, and the exposed lithium nickelcomposite oxide was subjected to the thermal decomposition-gaschromatography method to measure the amount of the resulting carbondioxide gas. The results are shown in FIGS. 2 and 3.

FIG. 2 shows the results corresponding to the conditions of 0 hours ofdry air exposure (no exposure), 3 hours of atmosphere exposure, 3 daysof atmosphere exposure, 5 days of atmosphere exposure, and 30 days ofatmosphere exposure. FIG. 3 shows the result of 10 days of atmosphereexposure. FIGS. 2 and 3 show that heating the atmosphere-exposed lithiumnickel composite oxide to 200° C. or higher generates carbon dioxidegas. FIGS. 2 and 3 also show that there is no significant difference inthe generation amount of the carbon dioxide gas between 400° C. heatingand 500° C. heating.

From these results, it can be said that the compound that is generatedwhen the lithium nickel composite oxide is exposed to atmosphere is acompound that is thermally decomposed by heating at 200 to 500° C. togenerate carbon dioxide gas. It is also seen that the generation amountof this compound substantially reaches its maximum at 400° C. Thiscompound that generates carbon dioxide gas will be hereinafter referredto as “thermally decomposing carbonate compound.”

FIGS. 2 and 3 also show that the longer the time for atmosphere exposureis, the amount of the carbon dioxide gas that is generated from the 400°C. heating tends to be larger. This is possibly because as the exposuretime becomes long, the amount of the resulting thermally decomposingcarbonate compound increases.

This result leads to the conclusion that the compound that is generatedwhen the lithium nickel composite oxide is exposed to atmosphere is notthe lithium carbonate.

When the lithium nickel composite oxide is synthesized as the positiveelectrode active material, lithium hydroxide is used as a lithiumsource. When the lithium hydroxide comes in contact with atmosphere, itpossibly reacts with carbon dioxide contained in atmosphere to generatelithium hydrogen carbonate. Since it is difficult to totally preventcontact with atmosphere in the whole production process of the lithiumnickel composite oxide, the existence of the lithium hydrogen carbonateis possibly inevitable. The lithium hydrogen carbonate that is generatedas a result of the reaction of the lithium hydroxide is decomposed bythe heat from the baking step to be rendered lithium carbonate, and itis possible that this lithium carbonate remains in the lithium nickelcomposite oxide. In experiment 5, a study was conducted on therelationship between the lithium hydrogen carbonate and carbon dioxidegas derived therefrom.

EXPERIMENT 5

When the lithium carbonate (Li₂CO₃) and the lithium hydrogen carbonate(LiHCO₃) react with hydrochloric acid, they generate carbon dioxide gasin the manners represented by formulas 1 and 2, respectively.Li₂CO₃+2HCl→2LiCl+H₂O+CO₂↑  Formula 1LiHCO₃+HCl→LiCl+H₂O+CO₂↑  Formula 2

When the lithium hydrogen carbonate is thermally decomposed, itgenerates carbon dioxide gas in the manner represented by formula 3.2LiHCO₃→Li₂CO₃+H₂O+CO₂↑  Formula 3

From formulas 1 to 3, when the thermally decomposing carbonate compoundis assumed to be the lithium hydrogen carbonate, the amount (mole) ofthe carbon dioxide gas that is generated from the 500° C. heating is ½the amount (mole) of the thermally decomposing carbonate compound. Also,the lithium hydrogen carbonate is decomposed by the baking heat (700°C.) from the synthesis of the lithium nickel composite oxide intolithium carbonate, water, and carbon dioxide. Hence, when the thermallydecomposing carbonate compound is the lithium hydrogen carbonate, thelithium carbonate amount can be calculated from formula 4 with theamount of the carbon dioxide gas that is generated from the hydrochloricacid treatment and the amount (mole) of the carbon dioxide gas that isgenerated from the 500° C. heating.{Lithium carbonate amount (number of moles)}={Amount of carbon dioxidegas generated from hydrochloric acid treatment (number ofmoles)}−{Amount of carbon dioxide gas generated from 500° C. heating(number of moles)}  Formula 4

The amount of the carbon dioxide gas that is generated during the 500°C. heating is thought to be the amount of the thermally decomposingcarbonate compound. On the basis of the above hypothesis, experiment 5was carried out in the following manner.

A lithium nickel composite oxide was prepared in the same manner as inexperimental example 1. This lithium nickel composite oxide was exposedunder various conditions and then allowed to react with hydrochloricacid, followed by measurement of the amount of carbon dioxide gas. Withthe assumption that the thermally decomposing carbonate compound was thelithium hydrogen carbonate, the amount of the thermally decomposingcarbonate compound and the amount of the lithium carbonate werecalculated from the above formula. The results are shown in FIG. 4.

FIG. 4 shows that the amount of the lithium carbonate is substantiallyconstant regardless of the exposure conditions, while the amount of thethermally decomposing carbonate compound is small at 0 days of exposurebut drastically increases as the degree of exposure increases.

The results of experiments 3 to 5 show that by measuring the amount ofthe carbon dioxide gas generated from heating to 200° C. or higher, theamount of the thermally decomposing carbonate compound contained in thelithium nickel composite oxide can be measured. Further, from FIGS. 2and 3, which show that the amount of the carbon dioxide gas generatedreaches its maximum at 400° C., it can be seen that heating to 400° C.or higher enables the accurate amount of the carbon dioxide gasgenerated to be determined.

EXPERIMENT 6

In view of the above knowledge, a lithium nickel composite oxideprepared in the same manner as in experimental example 1 was heated at“500° C.,” a temperature higher than 400° C., followed by measurement ofthe amount of the resulting carbon dioxide gas. At the same time, theswelling amount (after cooling) of a cell that used this lithium nickelcomposite oxide was examined. The result is shown in FIG. 5.

FIG. 5 shows that there is a linear-functional relation between thegeneration amount of the carbon dioxide gas and the cell swellingamount. From this result, the thermally decomposing carbonate compoundcontained in the positive electrode active material is possiblydecomposed during preservation at high temperature to generate carbondioxide gas, which in turn increases the cell swelling amount. Judgingcomprehensively from this result and the results of experiment 5 (FIG.4) and experiments 1 to 4 in light of the above hypothesis, it can besaid that the lithium carbonate contained in the lithium nickelcomposite oxide is derived from the lithium hydrogen carbonate generatedby the reaction between the lithium hydroxide serving as a lithiumsource and carbon dioxide gas contained in atmosphere. During synthesisby baking of the lithium nickel composite oxide, the lithium hydrogencarbonate is thermally decomposed into carbon dioxide gas, water, andlithium carbonate (see formula 3), and this lithium carbonate generatedhere remains in the lithium nickel composite oxide.

Thus, the main body of the thermally decomposing carbonate compound ispossibly the lithium hydrogen carbonate, and therefore, by knowing theamount of the carbon dioxide gas generated from heating the lithiumnickel composite oxide, it is possible to judge whether the lithiumnickel composite oxide is degraded in quality as a result of exposure toatmosphere. That is, the generation amount of the carbon dioxide gas atthe time of heating the lithium nickel composite oxide serves as anindicator in judgment for a suitable positive electrode active material.

This conclusion has support in Table 3. Table 3 incorporates thegeneration amount (FIG. 4) of the carbon dioxide gas in the case ofheating the lithium nickel composite oxide to 500° C.

TABLE 3 Amount of cell swelling *Generation (mm) amount of ImmediatelyInitial Discharge carbon Exposure after picking Charge Dischargeefficiency load property dioxide gas conditions out After coolingcapacity (%) capacity (%) (%) (%) (μmole/g) Experimental Example 1 None0.73 0.26 100.0 100.0 88.8 106.7 3.6 Experimental Example 2 Dry air: 100.92 0.36 99.6 100.1 89.1 106.6 7.9 days Experimental Example 3 Dry air:30 1.17 0.43 100.2 99.7 88.5 107.0 13.0 days Experimental Example 4Atmosphere: 1.12 0.46 100.5 100.4 88.9 106.9 8.0 3 Hrs ExperimentalExample 5 Atmosphere: 1.89 0.69 100.2 98.8 88.4 107.8 21.0 1 dayExperimental Example 6 Atmosphere: 3.97 1.25 99.0 96.9 88.1 108.2 37.0 3days Experimental Example 7 Atmosphere: 4.43 1.61 96.0 90.0 85.9 110.257.0 10 days Experimental Example 8 Atmosphere: 4.89 1.67 95.9 89.2 85.5110.5 64.0 10 days Dry air: 10 days *The case of heating lithium nickelcomposite oxide to 500° C.

Table 3 shows that there is a good correlation between the generationamount of the carbon dioxide gas and whether the various properties ofcell swelling amount, charge capacity, discharge property, and initialefficiency are good or bad. Table 3 also shows that experimentalexamples 7 and 8, which experienced exposure under the conditions of 10days or more, are significantly inferior in the various properties ofcell swelling amount, charge capacity, discharge property, and initialefficiency to experimental examples 1 to 6, which experienced exposureunder the conditions of 3 days or less. Hence, the generation amount ofthe carbon dioxide gas can be used as a judgment criterion (indicator)for quality control. For example, when experimental examples 7 and 8 areassumed to be defective products, a lithium nickel composite oxide witha carbon dioxide gas amount of 57.0 or more is judged as an unsuitableproduct (defective product) as the positive electrode active material,while a lithium nickel composite oxide with a carbon dioxide gas amountof less than 57.0 is judged as a suitable product (good product).

The criteria for judging quality can be generalized in the mannersrepresented by formulas 1 and 2 with the use of FIG. 3.y<(0.27x−51)/1000000(200≦x<400)  formula 1y<57/1000000(400≦x≦1500)  formula 2

where x is the heating temperature (° C.) and y is the amount of carbondioxide gas (mole/g) per 1 g of the lithium nickel composite oxide.

In the above formulas, raising the heating temperature beyond 1500° C.may decompose the lithium carbonate as well as the thermally decomposingcarbonate compound. In view of this, the upper limit of the heatingtemperature is 1500° C. The lower limit of the heating temperature is400° C. or higher, at which the generation amount of the carbon dioxidegas approximately reaches an equilibrium. A more preferable upper limitof the heating temperature is 500° C. considering heating costs.

<Judgment Method for the Quality of the Complete Positive Electrode>

The degradation of the quality of the lithium nickel composite oxideoccurs not only in the course of synthesis of the lithium nickelcomposite oxide but also in the production process of the positiveelectrode and after completion thereof. Hence, the complete cell may notexhibit desired performance even though a suitable lithium nickelcomposite oxide is selected and used as the positive electrode activematerial. This necessitates an easy judgment method for judging whetherthe complete positive electrode is good or bad, in addition to thejudgment method for the quality of the lithium nickel composite oxide.In view of this, a study was conducted on a positive electrode qualityjudgment method for judging whether the quality of the complete positiveelectrode is good or bad.

EXPERIMENT 7

A positive electrode was prepared in the same manner as in experimentalexample 1 and exposed to atmosphere under predetermined conditions for apredetermined period of time. Then, a positive electrode mixture layer(active material layer made of the positive electrode active material,binding agent, and conductive agent) was stripped off the positiveelectrode, and this sample was subjected to the thermaldecomposition-gas chromatography method. The results are shown in FIG.6.

Comparison between FIG. 2 and FIG. 6 clearly shows that in the cases ofthe same exposure conditions, the positive electrode mixture layerexperiences a more drastic increase in the generation amount of thecarbon dioxide gas than the lithium nickel composite oxide alone.

A possible cause of this is that in the step of completing the positiveelectrode by mixing the positive electrode active material, bindingagent, and solvent together, the lithium (or lithium ions) in thelithium nickel composite oxide reacts with moisture and carbon dioxidegas in dry air to cause a larger amount of thermally decomposingcarbonate compound to be generated. However, this cause alone cannotexplain the drastic increase in the carbon dioxide gas amount. Hence,the drastic increase in the carbon dioxide gas amount possibly resultsfrom polyvinylidene fluoride serving as the binding agent.

That is, the polyvinylidene fluoride has fluorine in its molecule andtherefore generates lithium hydrogen carbonate when thermallydecomposed. The lithium carbonate alone is not decomposed into carbondioxide gas even if heated to 500° C., whereas the positive electrodemixture layer contains polyvinylidene fluoride and generates hydrogenfluoride when heated. This hydrogen fluoride decomposes not only thethermally decomposing carbonate compound (LiHCO₃) remaining in thelithium nickel composite oxide but also the lithium carbonate (Li₂CO₃).This possibly increases the generation amount of the carbon dioxide gasdrastically.

Referring to the generation amounts of the carbon dioxide gas at 400° C.and 500° C. shown in FIG. 6 while focusing on the difference in thegeneration amount of the carbon dioxide gas between the positiveelectrode mixture layers of different exposure conditions, heating totemperatures beyond 400° C. does not increase the difference. A possiblereason why there is no increase in the difference is that because of theeffect of the hydrogen fluoride, the carbon dioxide gas derived from thethermally decomposing carbonate compound is exhausted due to the heatingto 400° C.

EXPERIMENT 8

In experiment 8, in view of the results on experiment 7, a study wasfurther conducted on the influence of the polyvinylidene fluoride on theoccurrence of the carbon dioxide gas using, as the positive electrodeactive material, lithium cobalt composite oxide, which is difficult tothermally decompose and generates substantially no carbon dioxide gas,lithium carbonate, and lithium nickel composite oxide that was not atall exposed to atmosphere.

<Preparation of Positive Electrode a>

Lithium carbonate and cobalt oxide were mixed together and baked at 700°C., thus obtaining lithium cobalt composite oxide (LiCoO₂).

Ninety five mass parts of the lithium cobalt composite oxide, 2.5 massparts of carbon powder as a conducting agent, 2.5 mass parts ofpolyvinylidene fluoride (PVdF) as a binding agent, andN-methyl-2-pyrrolidone (NMP) were mixed together, thus preparing apositive electrode active material slurry. This positive electrodeactive material slurry was applied to both surfaces of a positiveelectrode current collector made of aluminum and dried. Then, theresulting product was rolled with a compressive roller, thus preparing apositive electrode a. The preparing steps were carried out in anatmosphere of 43% relative humidity and 25° C. temperature.

<Preparation of Positive Electrode b>

Ninety mass parts of the lithium carbonate, 10 mass parts ofpolyvinylidene fluoride (PVdF) as a binding agent, andN-methyl-2-pyrrolidone (NMP) were mixed together, thus preparing aslurry. This slurry was applied to both surfaces of a positive electrodecurrent collector made of aluminum and dried. Then, the resultingproduct was rolled with a compressive roller, thus preparing a positiveelectrode b. The preparing steps were carried out in an atmosphere of43% relative humidity and 25° C. temperature. It should be noted thatthe lithium carbonate does not function as an active material.

<Preparation of Positive Electrode c>

A positive electrode c was prepared in the same manner as inexperimental example 1 except that the steps of preparing a positiveelectrode active material slurry containing lithium nickel compositeoxide containing aluminum, applying the slurry, drying the slurry, androlling this product were carried out in a dry air atmosphere having adew point of −40° C. or lower. The positive electrode c was prepared inan ideal environment (in the most preferable atmosphere), and theelectrode of experimental example 1 was prepared in a slightly inferiorenvironment in that the step of preparing the slurry and the step ofrolling the slurry were carried out in a dry air atmosphere, and theapplying step of the slurry and the drying step of the slurry werecarried out in an atmosphere of 43% humidity.

Positive electrode mixture layers were stripped off positive electrodesa and b, and the generation amount of carbon dioxide gas was measured bythe thermal decomposition-gas chromatography method. The measurement wascarried out to two samples of each of the examples. The results areshown in FIG. 7. The positive electrode mixture layer of positiveelectrode a was made of lithium cobalt composite oxide, a conductingagent, and a binding agent, while the positive electrode mixture layerof positive electrode b was made of lithium carbonate and a bindingagent.

The positive electrode mixture layers were stripped off positiveelectrode a, which used a positive electrode mixture made of lithiumcobalt composite oxide, a conducting agent, and a binding agent,positive electrode c, which used a positive electrode mixture made oflithium nickel composite oxide containing aluminum, a conducting agent,and a binding agent, and the positive electrode according toexperimental example 1, which differed from positive electrode c only inproduction conditions. For these positive electrode mixture layers, thegeneration amount of carbon dioxide gas was measured by the thermaldecomposition-gas chromatography method. The results are shown in FIGS.7 and 8.

FIG. 7 clearly shows that carbon dioxide gas occur from heating to 300°C. or higher in the cases of positive electrode a, which used as thepositive electrode active material a lithium cobalt composite oxidecontaining no thermally decomposing carbonate compound or, if any, aminute amount of thermally decomposing carbonate compound, and positiveelectrode b, which used lithium carbonate and contained no conductingagent.

FIG. 8 shows that carbon dioxide gas occur from heating to 300° C. orhigher in the case of positive electrode c, which contained no thermallydecomposing carbonate compound or, if any, a minute amount of thermallydecomposing carbonate compound and was prepared in a dry air atmosphere.

The figures also show that in all the cases of positive electrodes a toc, the higher the heating temperature becomes, the larger the generationamount of carbon dioxide gas tends to be. FIG. 8 also shows that in thetemperature range of 200° C. or higher and in the temperature range of400° C. or lower, the generation amount of carbon dioxide gas is largerin the positive electrode according to experimental example 1 than inpositive electrodes a and c.

These results lead to the conclusion that the conducting agent (carbonpowder) is not the cause of the carbon dioxide gas, and much of thecarbon dioxide gas that occurred in positive electrodes a to c isattributable to the existence of polyvinylidene fluoride.

The cause of the carbon dioxide gas in each of the positive electrodescan be analyzed in the following manner. First, description will be madeof lithium carbonate. The lithium carbonate alone does not generatecarbon dioxide gas even if heated to 500° C., as shown in FIG. 1. Hence,the occurrence of the carbon dioxide gas in positive electrode b, whichis mainly made of lithium carbonate, is because the lithium hydrogencarbonate generated from the polyvinylidene fluoride promoted thethermal decomposition reaction of the lithium carbonate.

Next, description will be made of positive electrode a, which useslithium cobalt composite oxide as the positive electrode activematerial. The lithium cobalt composite oxide is synthesized usinglithium carbonate as a lithium source, and the lithium carbonate doesnot react with moisture existent in the environment atmosphere. Thelithium cobalt composite oxide is less susceptible to moisture existentin the environment atmosphere than the lithium nickel composite oxide.The lithium nickel composite oxide itself is not decomposed attemperatures between 200 and 500° C. Thus, the carbon dioxide gas inpositive electrode a is caused by remaining lithium carbonate thatexperiences no reaction in the course of synthesis. That is, the carbondioxide gas is a result of thermal decomposition of the remaininglithium carbonate influenced by the hydrogen fluoride derived from thepolyvinylidene fluoride.

Description will be made of positive electrode c. The difference in thegeneration pattern of carbon dioxide gas between the positive electrodeaccording to experimental example 1 and positive electrode c is that inthe case of the former, the slurry applying step and the slurry dryingstep were carried out in an atmosphere of 43% humidity, while in thecase of the latter, even these steps were carried out in an idealatmosphere with moisture shut out. This difference between the positiveelectrodes is clearly observed during heating to 200 to 400° C. However,the difference is no longer observed when the heating reaches 500° C.Hence, in order to detect the difference, the heating temperaturecondition needs to be 200° C. or higher and 400° C. or lower.

Further, from FIG. 8, which shows that positive electrode c prepared inan ideal atmosphere and positive electrode a using lithium cobaltcomposite oxide, which is hardly influenced by moisture in atmosphere,have substantially the same generation amounts and generation patternsof carbon dioxide gas at 400° C. or lower, and from FIG. 2, which showsthat the generation amount of carbon dioxide gas is extremely small inthe cases of “-□-” (0 hours of dry air exposure) and “-∘-” (3 hours ofatmosphere exposure), it can be said that the generation of carbondioxide gas in the positive electrode according to experimental example1 and positive electrode c is due to the influence of the hydrogenfluoride derived from the polyvinylidene fluoride.

Thus, it can be seen that by estimating as a control the generationamount of the carbon dioxide gas in the case of heating to 200 to 400°C. the lithium cobalt composite oxide positive electrode (positiveelectrode a), in which the amounts of the components other than thepositive electrode active material are the same, the generation amountof the carbon dioxide gas in the case of heating to 200 to 400° C. thepositive electrode mixture layer of the lithium nickel composite oxidepositive electrode can be used as an indicator for judging the qualityof the lithium nickel composite oxide positive electrode. This wasfurther studied by referring to FIG. 9.

In the experimental result of the thermal decomposition of the lithiumnickel composite oxide positive electrode active material shown in FIG.3, the generation amount of the carbon dioxide gas in the case ofheating to 500° C. is approximately twice the generation amount in thecase of heating to 300° C. In view of this, the generation amounts ofthe carbon dioxide gas in the case of 500° C. for the lithium nickelcomposite oxide were picked from FIG. 2, and corresponding generationamounts of the carbon dioxide gas in the case of 300° C. for thepositive electrode mixture layer were picked from FIG. 6. Doubled valuesof the generation amounts of the carbon dioxide gas in the case of 300°C. for the positive electrode mixture layer and the generation amountsof the carbon dioxide gas in the case of 500° C. for the lithium nickelcomposite oxide are shown in corresponding pairs in FIG. 9. The same wascarried out to the cases of positive electrodes a and c, the results ofwhich are also shown in FIG. 9.

FIG. 9 shows that the relation between the exposure conditions and thedoubled values of the generation amounts of the carbon dioxide gasgenerated from heating the positive electrode mixture layer to 300° C.shows substantially the same tendency as the relation between theexposure conditions and the amounts of the carbon dioxide gas generatedfrom heating the positive electrode to 500° C. Hence, the amount of thethermally decomposing carbonate compound in the positive electrodemixture layer (the compound is decomposed when preserved at hightemperature to generate carbon dioxide gas, ending up as a causingsubstance of the increase in the amount of cell swelling and decrease inthe conductivity of the positive electrode) can be predicted using thegenerated amount of the carbon dioxide gas in the case of heating to200° C. or higher and 400° C. or lower.

FIG. 10 shows a relation between the generation amount of the carbondioxide gas in the case of heating the positive electrode mixture layerto 300° C. (see FIG. 6) and the amount of cell swelling. FIG. 10 clearlyshows that there is a linear-functional relation between the generationamount of the carbon dioxide gas in the case of the 300° C. heating andthe amount of cell swelling. Hence, by using the generation amount ofthe carbon dioxide gas in the case of heating the positive electrodemixture layer to 300° C. as an indicator, whether the quality of thepositive electrode is good or bad can be judged. It should be noted thatthe amount of cell swelling is a value measured after the cell is leftto cool to room temperature (25° C.).

Table 4 shows a list of the generation amounts (per 1 g of the positiveelectrode active material) of the carbon dioxide gas in the case ofheating the positive electrode mixture layer to 300° C. and the resultsof experimental example 1.

TABLE 4 Amount of cell swelling *Generation (mm) amount of ImmediatelyInitial Discharge carbon Exposure after picking Charge Dischargeefficiency load property dioxide gas conditions out After coolingcapacity (%) capacity (%) (%) (%) (μmole/g) Experimental Example 1 None0.73 0.26 100.0 100.0 88.8 106.7 43 Experimental Example 2 Dry air: 100.92 0.36 99.6 100.1 89.1 106.6 52 days Experimental Example 3 Dry air:30 1.17 0.43 100.2 99.7 88.5 107.0 63 days Experimental Example 4Atmosphere: 1.12 0.46 100.5 100.4 88.9 106.9 — 3 Hrs ExperimentalExample 5 Atmosphere: 1.89 0.69 100.2 98.8 88.4 107.8 — 1 dayExperimental Example 6 Atmosphere: 3.97 1.25 99.0 96.9 88.1 108.2 99 3days Experimental Example 7 Atmosphere: 4.43 1.61 96.0 90.0 85.9 110.2135  10 days Experimental Example 8 Atmosphere: 4.89 1.67 95.9 89.2 85.5110.5 138  10 days Dry air: 10 days *The case of heating positiveelectrode mixture layer to 300° C. —: Not measured.

Table 4 shows that when experimental examples 7 and 8 are assumed to bedefective products while experimental examples 1 to 6 are assumed to begood products, then the 135 μmole/g carbon dioxide gas can be assumed asa reference such that a product is judged as a defective product whenthe generation amount of the carbon dioxide gas is equal to more thanthe reference or as a good product when the generation amount is lessthan the reference.

In view of this, the judgment reference is generalized on the basis ofFIG. 6. The case of satisfying formulas 3 and 4 is judged as a goodproduct (suitable product), while the case of not satisfying formulas 3and 4 is judged as a defective product (unsuitable product).y<(1.31x−258)/1000000(200≦x<300)  formula 3y<1.20x−225/1000000(300≦x≦400)  formula 4

where x is the heating temperature (° C.) and y is the generation amountof carbon dioxide gas (mole/g) per 1 g of the positive electrode activematerial.

EXPERIMENT 9

In experiment 9, a study was conducted on the heating atmosphereconditions. A lithium nickel composite oxide was prepared in the samemanner as in experimental example 1. Then, a positive electrode mixturewas prepared using this lithium nickel composite oxide and applied ontoa core material of the positive electrode mixture, and then dried. Thisproduct was then rolled, thus preparing a complete positive electrode.Then, the lithium nickel composite oxide and the positive electrodemixture removed out of the complete positive electrode were put intoargon gas-filled SUS (Stainless Used Steel) reaction tubes and dryair-filled SUS (Stainless Used Steel) reaction tubes. These SUS reactiontubes were heated to measure the amount of the resulting carbon dioxidegas.

The results are shown in FIGS. 11 and 12. The measurement for thelithium nickel composite oxide under the 500° C. heating temperaturecondition was carried out using three samples in the both cases of dryair and argon gas. In the case of the positive electrode mixture layer,the measurement was carried out using two samples only in the case ofargon gas at the 500° C. heating temperature (o, A in FIG. 12).

FIG. 11 shows that in the conditions of 500° C. heating temperature anddry air, the three measurement values vary considerably (see ●, ▪, ▴).On the contrary, in the conditions of 500° C. heating temperature andargon gas, the values show a small variation (see ∘, □, Δ). Thegeneration amount of carbon dioxide gas is large in the dry aircondition and small in the argon gas condition. FIG. 12 also shows thistendency in the case of the positive electrode mixture layer.

FIGS. 11 and 12 show that the generation amount of carbon dioxide gas inthe dry air atmosphere is much larger than in the argon atmosphere bothin the case of the lithium nickel composite oxide and the positiveelectrode mixture layer. The largest difference between the dry airatmosphere and the argon gas atmosphere is the presence or absence ofactive gas. In the measurement in the argon gas atmosphere, where noactive gas such as oxygen exists, there is a small variation betweenmeasurement values and the increase in the generation amount of carbondioxide gas is also small. On the contrary, the dry air atmospherecontains oxygen derived from atmosphere. Hence, in the heating in thedry air atmosphere, an organic substance that remains in minute amountsin the lithium nickel composite oxide and the positive electrode mixturelayer, which are measurement samples, and in the test tubes (SUSreaction tubes) possibly reacts with this oxygen (combusts) to generatecarbon dioxide gas. This carbon dioxide gas possibly adds to the amountof the carbon dioxide gas that is derived from the thermally decomposingcarbonate compound and that the present invention originally intends tomeasure, thereby causing the carbon dioxide gas amount to increase andthe measurement values to vary.

From the results shown in FIGS. 11 and 12, it can be said that theorganic substance that remains in minute amounts in the lithium nickelcomposite oxide and the positive electrode mixture layer and in the SUS(Stainless Used Steel) reaction tubes is large compared with thegeneration amount of the originally intended carbon dioxide gas to bemeasured by the present invention. Hence, the quality judgment methodaccording to the present invention needs to thermally decompose thesample in an atmosphere using inactive gas (such as argon as andnitrogen gas). Argon gas is particularly preferable in that it ischemically inactive and non-reactive. Therefore, the thermaldecomposition is preferably carried out in an argon gas atmosphere.

(Supplemental Remarks)

The lithium nickel composite oxide encompassed by the present inventionis not limited to LiNi_(0.8)Co_(0.15)Al_(0.05)O₂. For example, thepresent invention encompasses Li_(a)Ni_(x)M_(1-x)O₂ (where M is at leastone selected from Co, Al, Zr, Ti, Mg, and Mn, 0.9≦a≦1.1, and 0.5≦x≦1).

(Industrial Applicability)

As has been described hereinbefore, with the quality judgment method ofpositive electrode active material according to the present invention,whether the quality of a positive electrode active material mainly madeof lithium nickel composite oxide is good or bad can be judged easily.Also, with the positive electrode quality judgment method according tothe present invention, whether the quality of a positive electrode usingthe positive electrode active material mainly made of lithium nickelcomposite oxide is good or bad can be judged easily before cellassembly.

The quality judgment methods according to the present invention findapplications in improvements in the synthesis conditions andpreservation conditions for the lithium nickel composite oxide. Also, byselecting and using a positive electrode active material judged as agood product by the positive electrode active material quality judgmentmethod according to the present invention, a high quality positiveelectrode can be prepared, and by selecting and using a positiveelectrode judged as a good product by the positive electrode qualityjudgment method according to the present invention, a non-aqueouselectrolyte secondary cell having desired performance is produced with agood yield. Thus, the industrial applicability of the present inventionis considerable.

1. A quality judgment method for a positive electrode for a non-aqueouselectrolyte secondary cell having a positive electrode mixture layercontaining a positive electrode active material having a lithium nickelcomposite oxide and polyvinylidene fluoride, the method comprising thesteps of: a sampling step of sampling the positive electrode mixturelayer from the positive electrode; a heating step of heating the sampledpositive electrode mixture layer to a temperature x (° C.) of 200° C. orhigher and 400° C. or lower; a measuring step of measuring the amount ofcarbon dioxide gas generated from the heating; and a judging step ofjudging the positive electrode as a suitable positive electrode for thenon-aqueous electrolyte secondary cell when the positive electrodeactive material satisfies formulas 3 and 4:y<(1.31x−258)/1000000(200≦x<300)  formula 3y<1.20x−225/1000000(300≦x≦400)  formula 4 where x is the heatingtemperature x (° C.) and y is the amount of carbon dioxide gas (mole/g)generated per 1 g of the positive electrode active material in theheating to the heating temperature x (° C.).
 2. The quality judgmentmethod for a positive electrode for a non-aqueous electrolyte secondarycell having a positive electrode mixture layer containing a lithiumnickel composite oxide and polyvinylidene fluoride according to claim 1,wherein the heating temperature x (° C.) is 300±50° C.
 3. The qualityjudgment method for a positive electrode for a non-aqueous electrolytesecondary cell having a positive electrode mixture layer containing alithium nickel composite oxide and polyvinylidene fluoride according toclaim 1, wherein the measurement of the amount of carbon dioxide gas inthe measuring step is carried out by gas chromatography.
 4. The qualityjudgment method for a positive electrode for a non-aqueous electrolytesecondary cell having a positive electrode mixture layer containing alithium nickel composite oxide and polyvinylidene fluoride according toclaim 1, wherein the heating is in an inactive argon gas atmosphere. 5.The quality judgment method for a positive electrode for a non-aqueouselectrolyte secondary cell having a positive electrode mixture layercontaining a lithium nickel composite oxide and polyvinylidene fluorideaccording to claim 1, wherein the mass percentage of the lithium nickelcomposite oxide in the positive electrode active material is 50 mass %or more.